[lldbtests] Handle errors instead of crashing.

If you pass an invalid compiler/debugger path on the cmdline to `dotest.py`  this is what you get.

  Traceback (most recent call last):
  [...]
    File "dotest.py", line 7, in <module>
      lldbsuite.test.run_suite()
  [...]

And with the patch applied:

  /home/davide/work/build-lldb/bin/clandasfasg is not a valid path, exiting

Differential Revision:  https://reviews.llvm.org/D39199

llvm-svn: 316393